The treatment of wastewater has historically been accomplished through anaerobic and aerobic processes. In the aerobic treatment of wastewater, a system exposes the wastewater to an oxygen atmosphere. A dissolved air flotation ("DAF") system is an example of a system using an aerobic process.
In a typical DAF system, recycled water or the raw wastewater is pressurized in a pressurization tank. Compressed air is then added to the water in the pressurization tank. This water, along with flocculating chemicals is introduced into the DAF vessel. Bubbles are formed as the air entrained in the water is released from pressure. The particles attach themselves to or become trapped by the bubbles. As the bubbles rise to the surface of the water, the particles are carried along. When the bubbles reach the surface the air escapes, forming a foam or sludge layer on the surface. The sludge layer contains the particles deposited by the escaping air bubbles. The sludge layer is kept intact and skimmed from the vessel to be processed by various means. Thus, through aeration, the wastewater is fed needed air and chemicals and the water clarified.
In an anaerobic process, digestion of the organic matter by gas forming bacteria occurs in a digester. Historically, anaerobic treatment of wastewater has not included aeration of the wastewater for further clarification where the use of oxygen or air to aerate the wastewater will cause the resultant sludge to become inactive since anaerobic treatment requires an oxygen-free atmosphere. In presently available anaerobic processes the wastewater is not well flocculated, making it difficult to separate solids from the liquor, resulting in less clarity than achieved in wastewater treated aerobically.